Sonar devices are frequently used by sport fisherman and boaters. These devices include means for generating high-frequency sound pulses and receiver/transducer means responsive to reflected sound pulses for acquiring target data such as the location of fish and underwater obstacles, and the depth of the bottom of the body of water. Typically, the sonar apparatus generates a series of pulses of sound at periodic intervals, receives reflected sound pulses or echoes from underwater objects, and displays either a depth readout of a target or the location of a target on a linear or curvilinear array scaled as to depth, which is a function of the time elapsed between the transmission of the sound pulses until reception of the reflected echo.
In prior art depth sounders, various methods are used to display the reflected sound pulses or "sonar returns" as a function of depth. Rotating disk lamps or "flashers", chart recorders, and numerical digital depth read-outs are commonly employed to indicate the depth of a sonar return. These types of displays suffer from various disadvantages which are either inconvenient or annoying to a user.
One problem with many prior art apparatus is that a user must interpret the display to determine the bottom of the body of water since the display is typically a linear or curvilinear scale which extends to the extent of the capability of the apparatus. The bottom return in some devices such as chart recorders typiclly appears as a wide area or band on the display, but conditions such as thermocline and multiple returns caused by reflection from the boat bottom or other sources create additional sonar returns indicated as occurring at depths above or below the actual bottom, tending to make interpretation more difficult.
Some prior art linear or curvilinear displays such as flashers are only one-dimensional, in that the linear or curvilinear display can only display information for a single transmission. A sonar return subsequent to a currently-displayed return is erased or overwritten by new information. If a moving target is detected, the target such as a fish may disappear from the display after the next transmission, and the user may miss the target if the display has not been constantly watched.
In order to overcome the disadvantages of a one-dimensional display, other prior art sonar apparatus employ a chart-printing device or a cathode ray tube (CRT) to provide a two-dimensional display which lowers the risk of missing targets. Some of these devices include scale-changing features which allow different depth scales to be selected and associated with the display. However, when a scale change is made, a discontinuity at the point of the scale change makes interpretation of the display difficult. For example, if a target is detected at 15 feet on a 60 foot nominal scale, and the scale of the display is changed to 120 foot nominal depth, the previously detected target at 15 feet will remain in the same relative position on the display, since the device cannot go back and "rewrite" what has previously been written. However, new returns for the same target at 15 feet will appear at a different location on the display in the 120 foot nominal depth scale. Thus, the target will appear to have shifted upwardly on the display. Discontinuities such as these create confusion in display interpretation.
Some prior art sonar apparatus include a scale expansion feature wherein the depth scale is expanded by a predetermined factor two, e.g. U.S. Pat. No. 4,322,827 to Weber. Other prior art devices include a scale expansion feature wherein an upper depth limit and a lower depth limit are keyed into the device, so that target data detected within these depth limits can be expanded to fill the display. Still other types of displays include prepicked scale expansion regions having a fixed number of fixed limit expansion regions.
All of these prior art approaches to display expansion are subject to criticism. The predetermined factor of two approach, while simple, cannot be used to "zero in" on a selected area for expansion. The selectable upper and lower limit approach requires the user to enter the limit data via a keypad, requiring mental calculations to determine the appropriate areas for display expansion. The prepicked expansion lies close to a boundary between expansion regions, so that selection of one expansion region followed by movement of the target requires the user to re-enter the expansion selection mode and select another expansion region.
Other problems exist in prior art sonar apparatus. A particular problem occurs in sonar apparatus having automatic gain control. For example, U.S. Pat. No. 4,420,824 to Weber entitled "SONAR APPARATUS HAVING IMPROVED GAIN CONTROL" discloses an apparatus wherein a microprocessor controls the gain of the receiver stage so that the gain of the receiver is automatically increased as the anticipated bottom depth increases. In this apparatus, the gain is set at a minimum at the time of and immediately succeeding a sonar transmission and as time increases, the receiver gain is increased in anticipatiion of weaker signals which correspond to greater depths.
A particular problem with variable gain amplifier circuits is that changing amplifier gain often creates transients which if not properly handled can appear as target returns. Typically, additional filters or other signal processing circuits required to suppress the transients or otherwise assure that the transients are not treated as a valid return signal.
Moreover, troubleshooting of variable gain amplifiers is difficult in that a repair technician is required to have detailed information as to expected outputs for a wide range of input signals. This typiclly entails employing a variety of input signal settings and adjustments in order to isolate a particular faulty component in a variable gain amplifier.
Another problem frequently encountered in marine sonar apparatus is providing a watertight enclosure to protect the electrical circuitry which still allows a convenient user interface. Individual waterproof switches are expensive and are still prone to leaks in that a separate seal for each of a plurality of switches increases the probability that one or more of the switch seals will fail under adverse climatic conditions. Significant improvement in weatherability could be obtained by minimizing the number of places requiring weatherproof seals.